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Developmental Psychology Unit 3: Infancy: Physical and Cognitive Development

The first two years bring the fastest brain and body growth of the lifespan, along with the leap from reflexes to symbolic thought and first words. This unit covers infant brain, motor, perceptual, cognitive and language development.

Covers: Brain development and plasticity, Reflexes, growth and motor milestones, Perception and research methods, Piaget's sensorimotor stage, Infant memory and core knowledge, Early language development

Study guide

At birth the brain has nearly all the neurons it will ever have, but it is only about a quarter of its adult weight. Growth comes from synaptogenesis (a burst of new connections), myelination (fatty sheaths that speed signals) and synaptic pruning, in which unused connections are eliminated, following a 'use it or lose it' rule. Experience-expectant growth depends on experiences nearly every infant has, such as patterned light and speech sounds; experience-dependent growth reflects experiences unique to each child. Plasticity is greatest early in life. Infants also follow growth patterns: cephalocaudal (head to tail, so head control comes before walking) and proximodistal (center outward, so arm control comes before finger control). A typical baby doubles birth weight by about 5 months and triples it by age 1.

Newborns arrive with reflexes. Rooting (turning toward a touch on the cheek) and sucking help with feeding; the Moro reflex is a startle in which the arms fling out and then pull in; the Babinski reflex makes the toes fan out when the sole is stroked; the palmar grasp curls the fingers around an object. Most of these fade in the first months as the cortex matures, and their persistence can signal neurological problems. Motor milestones follow a typical order: holding the head up, rolling over, sitting without support (about 6 months), crawling, standing with help, and walking alone (about 12 months), though the timing varies widely. Esther Thelen's dynamic systems theory argues that motor skills emerge as infants combine maturation, body growth, motivation and environmental opportunities.

Researchers study infant perception with clever methods. In preferential looking, infants look longer at what they prefer, such as faces or patterns. In habituation, looking declines as a stimulus becomes familiar; a jump in looking to a new stimulus (dishabituation) shows that the infant can tell the two apart. Newborn vision is blurry (around 20/400) but improves quickly; hearing is well developed at birth, and newborns prefer their mother's voice. Gibson and Walk's visual cliff showed that crawling infants avoid an apparent drop, and later work linked depth fear to crawling experience. Perceptual narrowing means infants lose the ability to tell apart sounds or faces not common in their environment by about 10-12 months, a sign that experience tunes perception.

Piaget called infancy the sensorimotor stage, in which infants think through senses and actions. Its six substages move from reflexes, to primary circular reactions (repeating actions on the body, like thumb sucking), secondary circular reactions (repeating actions that affect objects, like shaking a rattle), coordination of schemes with intentional goals (8-12 months), tertiary circular reactions (experimenting with variations, 12-18 months), and finally mental representation (18-24 months), shown in deferred imitation and pretend play. Object permanence, knowing objects exist when unseen, appears gradually; around 8-12 months infants make the A-not-B error. Renée Baillargeon's violation-of-expectation studies suggest that young infants understand more about objects than Piaget believed, supporting core-knowledge views.

Language develops on a predictable schedule. Infants coo (vowel sounds) by about 2 months, babble consonant-vowel strings by about 6 months, say first words around 12 months, show a vocabulary spurt around 18 months and combine two words in telegraphic speech by about 18-24 months. Early word use includes overextension (calling all four-legged animals 'doggie') and underextension (using 'cup' only for one's own cup). Receptive language, understanding, runs ahead of productive language. Skinner explained language through reinforcement, while Noam Chomsky proposed an innate language acquisition device; most researchers today take an interactionist view. Child-directed speech, with high pitch, slow pace and exaggerated tones, holds infant attention, and the amount of responsive talk a child hears predicts later vocabulary.

Key ideas

• Brain growth in infancy involves synaptogenesis, myelination and pruning of unused connections.
• Growth follows cephalocaudal and proximodistal patterns, and motor milestones follow a typical order with wide variation in timing.
• Most newborn reflexes fade within months, and their persistence can signal a neurological problem.
• Habituation and preferential looking let researchers infer what infants can perceive and remember.
• Piaget's sensorimotor stage progresses from reflexes to mental representation by about age 2.
• Violation-of-expectation research suggests infants understand object permanence earlier than Piaget claimed.
• Language moves from cooing to babbling to first words to two-word telegraphic speech, with comprehension ahead of production.

Formulas and rules

Common mistakes

• Thinking the brain grows mainly by adding neurons after birth; growth comes mostly from connections, myelin and glial cells.
• Mixing up primary circular reactions (actions on the body) with secondary circular reactions (actions on objects).
• Assuming that infants who make the A-not-B error have no object permanence at all.
• Treating overextension and underextension as the same error.
• Interpreting renewed looking after habituation as boredom rather than as detection of something new.

Before the test, make sure you can

• Explain synaptogenesis, pruning, myelination and plasticity.
• Identify newborn reflexes and describe the order of major motor milestones.
• Explain how habituation and preferential looking reveal infant perception.
• Identify each sensorimotor substage from a description of infant behavior.
• Describe object permanence, the A-not-B error and the violation-of-expectation challenge to Piaget.
• Order the milestones of early language and give examples of overextension and underextension.
• Compare learning, nativist and interactionist views of language.
• Calculate corrected age for a preterm infant.

Words to know

Synaptic pruning

The elimination of unused neural connections, which makes the remaining circuits more efficient.

Myelination

The coating of axons with myelin, which speeds the transmission of neural signals.

Experience-expectant growth

Brain development that depends on experiences nearly all infants have, such as seeing light and hearing speech.

Cephalocaudal pattern

Growth and motor control that proceed from the head downward.

Proximodistal pattern

Growth and motor control that proceed from the center of the body outward.

Moro reflex

A newborn startle response in which the arms fling outward and then pull in after a sudden noise or loss of support.

Babinski reflex

The fanning of a newborn's toes when the sole of the foot is stroked.

Dynamic systems theory

Thelen's view that motor skills emerge from the combination of maturation, body, motivation and environment.

Habituation

Reduced attention to a stimulus after repeated exposure.

Visual cliff

Gibson and Walk's apparatus showing that crawling infants perceive depth and avoid an apparent drop.

Perceptual narrowing

The loss, by about 10-12 months, of the ability to distinguish sounds or faces not common in one's environment.

Object permanence

Understanding that objects continue to exist when they cannot be seen or heard.

A-not-B error

Searching for a hidden object where it was found before rather than where it was last hidden.

Deferred imitation

Copying an action observed earlier, a sign of mental representation.

Violation of expectation

A method that measures longer looking at impossible events to infer what infants expect.

Overextension

Using a word too broadly, such as calling every man 'daddy'.

Telegraphic speech

Early two- or three-word sentences that leave out small function words, such as 'more milk'.

Child-directed speech

High-pitched, slow, exaggerated speech that adults use with infants.

Sample practice questions

30 more questions like these, from level 1 to 5, are in the practice set.